It is well known to prepare an electrically-semiconductive polyimide by incorporating an electrically-conductive carbon black and to mold the electrically-semiconductive polyimide in the form of a film (sheet, tube, etc), for example, for using the film for various types of applications. Conventionally, in the production process, a liquid composition of poly(amic acid) (hereinafter referred to as "conventional liquid composition") is first prepared by synthesizing poly(amic acid), which is a precursor of polyimide, in an organic polar solvent, followed by adding thereto the electrically-conductive carbon black to be mixed therewith. Viscosity of the resultant liquid composition can be adjusted by adding the organic polar solvent to suit preferable molding conditions. Thereafter, the liquid composition is used for molding a film, for example, wherein the following two steps are necessary. In the first step, the liquid composition is molded under the molding conditions where the poly(amic acid) used as a main ingredient is not imidated (the molding temperature is less than 250.degree. C.) and by desired means to obtain the molded product in a desired form. The organic polar solvent contained in the liquid composition is removed by evaporation to give a solid poly(amic acid) film containing the carbon black. In the second step, the poly(amic acid) film is gradually heated until the temperature reaches about 350.degree. C. so that the imidation is complete to give an electrically-semiconductive polyimide film with removing the remaining solvent by evaporation, thereby finishing the production process.
The inventors of the present invention have filed a number of patent applications in connection with the techniques explained above. However, during various types of research for improving the techniques, the inventors have found the following problems which should have been resolved at once.
One of the problems relates especially to the conventional liquid composition. The inventors found that the conventional liquid composition sharply changes in the electrical resistivity day by day when stored in ordinary state (at ordinary temperature and under atmospheric pressure). The electrical resistivity changes with time; it decreases in some cases and increases in other cases. The change in the electrical resistivity makes it impossible to obtain a desired molded product having a desired electrical resistivity unless the liquid composition is subjected to a molding immediately after the production. Because of the unstable electrical resistivity, the liquid composition cannot be prepared in a large scale to be stored and used in such a manner that a portion required for the production process is taken out of the stock. That is, a mass-production of the liquid composition is practically impossible.
Another problem is a nonuniform electrical resistivity of a molded product. It is possible to produce a molded product having a desired electrical resistivity from the unstable liquid composition if the liquid composition is molded immediately after the preparation. However, depending on use conditions of the molded product (for example, a long-term use with repetitive electrification and destaticization under a high voltage, such as an intermediate transfer belt in a color copying machine; a long-term use under a high temperature and high humidity; etc.), variations in the electrical resistivity occur, thereby preventing the molded product from maintaining the uniformity in the electrical resistivity once applied to the molded product.
The inventors carried out an extensive research to solve the above problems at once, and found a novel electrically-semiconductive poly(amic acid) composition which has a higher storage stability and maintains a stable electrical resistivity as being molded into a molded product, thereby to accomplish the present invention. The invention is easily achieved as described below.